Phenolic resins have been used for a long time in the wood materials industry for producing relatively weather-resistant particle boards having perpendicular tensile strength of V-100 quality (German Industrial Standards DIN 68,763). These resins are typically aqueous, alkaline-curing phenol-formaldehyde condensates produced by reacting phenol and formaldehyde in the presence of alkali at a phenol/formaldehyde mole ratio of 1:1.5 to 3.5, preferably 1:1.8 to 3, the quality of the product being mainly determined by the solids content, the viscosity, and the alkali content.
The higher the molarity of these resins, the better the bonding strength and the shorter the curing time and, consequently, the shorter the pressing time for the particle boards. With predetermined solids contents and viscosities, the size of the molecules or the degree of condensation is strongly dependent on the alkali content, i.e. the greater the amount of alkali phenolate present during the reaction, the higher the degree of condensation of these resins.
To lower the hygroscopicity of phenolic resin bound particle boards and to minimize deterioration due to moisture, it has been attempted (more intensely in recent years) to lower the alkali content of the particle board by lowering the alkali content in the resins. Hence, for producing particle boards, it has become customary in recent years to replace the strongly alkaline, 45 to 48% phenolic resins containing about 10 to 12% sodium hydroxide by low-alkaline resins with 5 to 8% NaOH. However, these low alkaline resins have a lower degree of condensation and a poorer bonding strength due to the high degree of penetration into the wood fiber, and a lower reactivity which requires longer pressing times. Moreover, by lowering the alkali content, the resins become less economic, because the price of alkali is less than that of phenol and formaldehyde.
Furthermore, to optimally utilize the particle board production facilities working at high operating cost, oligomeric diphenylmethane-4,4'-diisocyanate (MDI) and acidic-curing mixed condensates based on phenol, melamine, and urea have been used to a growing extent in recent years as bonding agents in addition to the alkaline-curing phenolic resins for gluing conditionally weather-resistant particle board, because they have short pressing times. Despite the considerably higher cost for these bonding agents, the production of particle board is economic because the pressing time is shorter. However, the use of MDI presents difficulties, among others due to its high tackiness to the pressing sheets, and involves additional cost.
The phenolic resins described in W.German specification No. 29 44 178 have a degree of condensation which can be maintained at the same high level as that of bonding agents with a higher alkali content by lowering the solids content by dilution with water to 30 to 41% without changing the viscosity. However, as a result of their low solids content, these resins introduce an undesirably high level of moisture into the particle board. It is necessary either to dry the particles to a higher degree than is usual, or to extend the pressing times in order to vaporize the water in the particle board.
The attempts to use as bonding agents resins with a low alkali content, but having the same high degree of condensation as bonding agents with a high alkali content (without lowering the solids content of the resins) have failed up to now, because as is known, water and aqueous alkali are the best solvents for such phenolic resins.
Therefore, it is one object of the invention to develop a low-priced phenolic resin with a low alkali content, but with a high degree of condensation and a high solids content, which is suitable as a sole bonding agent for producing the outer layers of particle boards, or possibly in combination with other bonding agents for producing the center layers of particle boards. Other objects will be apparent to those skilled in the art.